Computer tomography (CT, also called computed tomography) has evolved into a commonly used means, when it comes to generating a three-dimensional image of the internals of an object. The three-dimensional image is created based on a large number of two-dimensional X-ray images taken around a single axis of rotation. While CT is most commonly used for medical diagnosis of the human body, it has also been found applicable for non-destructive materials testing. Detailed information regarding the basics and the application of CT, can be found in the book “Computed Tomography” by Willi A. Kalender, ISBN 3-89578-216-5.
One of the key innovative aspects in future CT and X-ray imaging is the energy-resolved counting of the photons which are let through or transmitted by the object being analyzed when being exposed to X-ray radiation. Depending on the number and energy the transmitted photons have, it can be concluded, after a slice image reconstruction step, through which types of material the X-ray beams have traveled. In particular, this allows to identify different parts, tissues and materials within a human body.
When the detection or counting of photons is referenced, it is understood, that when a photon impinges on the conversion material of a sensor, it creates a charge pulse. This charge pulse (sometimes also referred to as current pulse) is detected and the presence of a photon is concluded. The charge pulse results from a larger number of electron-hole pairs, which arc generated, when an X-ray photon interacts with the sensor conversion material. The duration of this current pulse corresponds to the so-called charge collection time.
Detection of single electron-hole pairs is not in the focus of this application, but the processing of a charge pulse resulting from electron-hole pairs representing a photon, which is sometimes expressed by the formulations “detecting photons” or “counting photons”.
One of the main concerns when implementing a counting detector for computer tomography applications is to deal with the irregular nature of the arrival time sequence of the incoming photons. The flux of photons, which has to be considered, is very high and randomly distributed in time. The distribution of the photons can be described by a Poisson distribution, i.e. the arrival times are negative exponentially distributed.
Another concern comes in the context of the very high counting rates that are inseparably linked with computer tomography. Since count rates of photons in the range of 109 cps/mm2 (counts per second/mm2) have to be dealt with for the direct beam or a beam which just touches the object's surface, very fast counting detectors have to be employed. Typically, however, detectors that can operate at high count rates, suffer from increased noise (due to the short shaping time available to shape an electric pulse) and from a reduced signal amplitude (due to the ballistic deficit).